In order to produce ceramic and refractory bodies, ceramic particles of various sizes are mixed together to obtain a particle size distribution within the mix which will produce the desired density, porosity, and uniformity in the fired product. When these particles are mixed with water, for example, for slip casting or extrusion, the different sizes segregate or settle thus disrupting the particle size distribution and altering the desired density, porosity, and uniformity of the fired end product. This segregation or settling occurs when the slurries are standing before being fired or frozen according to the particular process being used. This settling or segregation of the particles is even more pronounced when the slurry is subjected to a vacuum or vibration operation to remove any entrained air introduced into the slurry during the mixing operation.
In slip casting, the particles are generally mixed with about 10 weight percent water together with a binder, such as feldspar or kaolin clay, in order to impart a green strength to the ceramic before firing. The slurry is then placed in a porous mold which absorbs most of the water contained in the slurry. This removal of water generally imparts a sufficient green strength to the resultant ceramic so that it can be subsequently fired. During this process, the coarse and fine particles segregate or settle to various extents, thus interfering with the uniformity of the resulting fired product.
This particle segregation or settling is even more pronounced in the freeze-cast process where the ceramic particles are intermixed with a colloidal silica sol to form slurries containing approximately 10 weight percent water. In this freeze-cast process, the slurry is introduced into a solid mold which is generally metallic, so that no water is removed from the slurry as would be from the porous molds which are used in the slip-casting process. If these slurries are allowed to stand for too long a period of time before being frozen, the particles segregate or settle and the slurry becomes watery. At times, the particles can settle to the extent that a layer of water can be observed on the top of the ceramic particles contained in the mold. This segregation or settling of particles can also occur during the operation where the temperature of the slurry is lowered to near freezing and/or while supercooling the slurry before it is frozen, since reducing the temperature of the entire slurry, particularly if large ceramic articles are to be produced, can be somewhat time consuming.
In addition, in both the conventional and freeze-cast techniques, the green strength of the casting is reduced by this particle segregation or settling.
The flow properties of the slurry are also affected by this particle segregation to the extent that, at least in part, it affects the manner in which the slurry is handled, such as by extrusion into molds.